High Magnetic Field Science and Its Application in the United States Current Status and Future Directions (2013) / Chapter Skim
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7 Magnet Technology Development
7 Magnet Technology Development
Pages 122-150
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From page 122... ...
For other high-field applications such as high energy physics accelerators, magnetic confinement fusion, and medical applications, the most advanced systems require large-scale superconducting magnets. Superconducting magnets permit the generation of very large magnetic field volumes with minimal electrical power 122
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From page 123... ...
Thus, to achieve higher magnetic fields, the required electrical power and cooling power must also be substantially increased
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From page 124... ...
There are many technical challenges and considerations for high field superconducting magnets, including the following: Critical current density is a function of field, temperature, strain, mechanical strength, and wire piece length. Cost and availability have become a major issue particularly when evaluating the trade-offs between high-temperature supercon ductors (HTS)
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From page 125... ...
The highest field NMR magnets are subcooled to ~2 K to achieve a higher critical current density, but this requires an even more complex cryogenic system, because helium becomes a superfluid below 2.2 K at atmospheric pressure. To achieve magnetic fields higher than 24 T from an all-superconducting magnet, it will be necessary to utilize a HTS conductor for the portion of the coil operating in higher magnetic fields.
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From page 126... ...
Presently these conductors can be made only as thin flat tapes. They exhibit a strong anisotropic critical current density in the presence of transverse magnetic fields.
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From page 127... ...
Magnet Technology Dvelopment 127 pnictide and chalcogenide compounds that soon followed. In the classification by structure, the six families that have been discovered to date are known as 11, 111, 1111, 122, 32522, and 42622 compounds [1-13]
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From page 128... ...
The availability of high magnetic fields, particularly the 45 T hybrid dc field magnet in addition to the pulsed-field magnets at LANL, played a critical role in the exploration of many of the interesting characteristics exhibited by these super conductors. Experimental evidence of multiband superconductivity also quickly revealed that the higher-critical-temperature members of this new class of uncon ventional superconductors exhibited very high upper critical fields comparable to the cuprates.
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From page 129... ...
TABLE 7.1 Critical Temperature, Upper Critical Field, and Structure Classification for Iron-Based Superconductors Hc2 II ab(T)
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FIGURE 7.4 Upper critical field phase diagram of several superconductors having potential for com mercialization in high field magnets. SOURCE: Courtesy of David Larbalestier, Florida State University/ NHMFL.
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From page 131... ...
Resistive magnets require high power to reach high field and high stored energy in the pulsed power source to achieve long, peak field durations. Here 100 ms is considered a long time.
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From page 132... ...
Superconducting Magnets The 2005 NRC report of the Committee on Opportunities in High Mag netic Field Science (COHMAG) included recommendations for specific magnet
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From page 133... ...
into scientific operation having been achieved several years ago, magnet technology R&D programs have been reoriented to achieving even more powerful superconducting beam bending and focusing magnets. The dipole magnets will likely operate with peak fields well beyond 12 T and strong focusing magnets with high-field gradients operating at similarly higher peak fields in the windings.
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From page 134... ...
This imposes a significant additional step in the integrated magnet fabrication of the conductor, coil, and insulation, which requires important changes in manufacturing processes and handling from those used for past accelerators. Operation at higher magnetic fields leads also to increased stored energy and magnetic forces necessitating stronger and better-integrated structural reinforcing materials.
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From page 135... ...
Although most of the magnetic fusion experiments built in the past and still operating use pulsed, adiabatic resistive magnets recooled with either water or liquid nitrogen, several new superconducting devices have come into operation in the past few years. A working fusion power reactor has always been envisaged to require the use of large-scale superconducting magnets, since creating very high fields over large volumes would make resistive magnets
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From page 136... ...
The W-7X is shown during construction in Figure 7.5 as well as one of its convoluted superconducting magnets. The largest and most significant large-scale application of superconducting magnets for magnetic confinement fusion is the ITER project.
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The present magnet program is focused on developing high current, high field, and HTS conductors for fusion magnets. This requires development of 50 kA class conductors that can operate in magnetic fields in the range 16-20 T
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(Right) One of the superconducting magnets comprising the complicated main confinement magnetic field.
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From page 139... ...
Proton radiotherapy's main use is for treating tumors where surrounding tissue has a low ionizing radiation tolerance. This is often the case for childhood cancers and for tumors near the eye, the spinal cord, or in the brain.
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The two largest outer ring poloidal field magnets are 24 m in diameter, each weighing 300 metric tons. SOURCE: © ITER Organization, http://www.iter.org/.
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From page 141... ...
A significant part of this high cost is due to the very large size, mass, and cost of the conventional cyclotron, the long beam transport system, and the huge rotatable gantry required to direct the proton beam to the patient. The use of superconducting cyclotrons begins to address the size and cost issue by reducing
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From page 142... ...
Thus cyclotrons can be made very compact by going to high magnetic fields. Currently two superconducting cyclotrons have been built for proton radiotherapy and are treating patients on a regular basis (MSU, 1993; Miyata et al., 2012)
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These charged particles have heavier mass than a single proton and thus require more powerful particle accelerators to achieve effective treatment energies. Ongoing activities include efforts in Japan at the Heavy-Ion Medical Accelerator in Chiba (HIMAC)
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From page 144... ...
This virtuous cycle is nowhere more evident than in the bootstrap process by which new magnets are themselves developed, where access to higher magnetic fields provides the means for testing and improving the new concepts and components that will make possible the next generation of magnets. It is imperative that magnet technology be constantly challenged -- and also supported!
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From page 145... ...
As the committee has discussed elsewhere in this report, the United States has largely ceded leadership in constructing high-field superconducting magnets for NMR to Europe, where there is a closer relationship between the national labs that provide the required technology and the companies that will build these magnets. In part, this reflects a long-term underfunding of both magnet technology research in this country and the research in high-strength materials that underlies this important area.
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From page 146... ...
Finding: The veritable explosion of new materials with new functionalities that we have witnessed in the past decade is a potent driving force for the need to push experimentation to higher fields, where new phases and new behaviors are invariably found. Although pulsed fields will always provide the highest peak fields, many of the most revealing measurement techniques have inher ent timescales or sensitivity requirements that make them practical only in constant magnetic fields.
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From page 147... ...
IEEE Transactions on Applied Superconductivity 21:2092. Bruker Biospin Corporation, International Workshop on High Field Solution and Solid State Bio molecular NMR, Les Houches, June 1, 2009, press release, http://www.bruker-biospin.com/ pr090601.html.
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From page 148... ...
2012. FeSe superconducting tapes with a high critical current density fabricated by diffusion method.
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From page 149... ...
Maeda, 2011, Epitaxial growth of FeSe0.5Te0.5 thin films on CaF2 substrates with high critical current density. Applied Physics Express 4:053101.
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From page 150... ...
2012. High intergrain critical current density in fine-grain (Ba0.6K0.4)
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